Tong and colleagues looked at Australia's incidence of hepatitis A from January 1991 to December 2000 and how atmospheric pressure seesawed or oscillated over the same period.

Specifically, the researchers looked at the Southern Oscillation Index (SOI), a measure of how atmospheric pressure between Darwin and Tahiti changes, a figure used to calculate the El Niņo Southern Oscillation (ENSO) phenomenon.

"Our results show that the SOI is significantly associated with the transmission of hepatitis A," Tong told ABC Science Online. "But why El Niņo events are associated with transmission of hepatitis A, the mechanism, remains to be determined."

Hepatitis A is a disease that affects the liver and is caused by the hepatitis A virus, which is passed from person-to-person via contaminated food or water.

The researchers found that the incidence of hepatitis A cases in Australia rose a month after a dip in the SOI, indicating warm and dry, drought-causing conditions.

Tong said that the El Niņo Southern Oscillation had previously been related to vector-borne and water-borne disease such as cholera and diarrhoea, and deaths from natural disasters, such as floods, bushfires and cyclones.

He said while scientists did not know exactly how the weather pattern could cause an increase in hepatitis A, it was well demonstrated that warmer weather conditions caused more transmission of food-borne disease.

But a 1997 Sydney outbreak of hepatitis A, linked to eating contaminated oysters, did not seem to be related to the SOI pattern.

"Our hypothesis is that other factors were responsible for that particular outbreak," said Tong.

He said the temperature during that year was quite high and this was not reflected in the SOI because the index only reflected 40% of temperature and rainfall patterns.